Motors, Part 15

Across the line starts of large motors can wreak havoc with the distribution system in a facility. In fact, they usually do. If you have large motors in your facility and they start across the line, look at the power signatures at various points in the facility. Even on different feeders, you may see significant effects.

Using a power conditioner to solve this doesn’t solve it but does introduce other problems. Instead, solve this problem at the source. The appeal of a power conditioner is “install a box and solve the problem.” While this problem really isn’t solved with that approach, it is solved when you install a soft start or a motor drive.

Both of these devices give you the ability to begin the motor turning slowly (starting softly), letting it gradually come up to speed. The main operational difference is that a motor drive also gives you the ability to vary the speed during operation. That extra ability isn’t free, so if the motor will operate at a constant speed then a soft start is the better choice.

Both of these save energy, reduce wear on mechanical parts, eliminate power anomalies caused by across the line starting, and extend motor life. Most models come with harmonics correction and power factor correction, further reducing energy costs. Note that power factor correction at the service is done to avoid a penalty from the utility. To actually save energy, you must do this correction at the load.